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</p><ol><li>延迟运算</li><li>渐进式取值</li></ol><h2 id="延迟运算"><a href="#延迟运算" aria-hidden="true" class="header-anchor">#</a> 延迟运算</h2><p>延迟运算很好理解，所有 Observable 一定会等到订阅后才开始对元素做运算，如果没有订阅就不会有运算的行为</p><pre class="language-javascript"><code><span class="token keyword">var</span> source <span class="token operator">=</span> Rx<span class="token punctuation">.</span>Observable<span class="token punctuation">.</span><span class="token keyword">from</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">,</span><span class="token number">4</span><span class="token punctuation">,</span><span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">var</span> example <span class="token operator">=</span> source<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>x <span class="token operator">=&gt;</span> x <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><p>上面这段代码因为 Observable 还没有被订阅，所以不会真的对元素做运算，这跟数组的操作不一样，如下</p><pre class="language-javascript"><code><span class="token keyword">var</span> source <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">,</span><span class="token number">4</span><span class="token punctuation">,</span><span class="token number">5</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">var</span> example <span class="token operator">=</span> source<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>x <span class="token operator">=&gt;</span> x <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span> 

</code></pre><p>上面这段代码执行完，example 就已经取得所有元素的返回值了。</p><p>延迟运算是 Observable 跟数组最明显的不同，延迟运算所带来的优势在之前的文章也已经提过这里就不再赘述，因为我们还有一个更重要的差异要讲，那就是<strong>渐进式取值</strong></p><h2 id="渐进式取值"><a href="#渐进式取值" aria-hidden="true" class="header-anchor">#</a> 渐进式取值</h2><p>数组的 operators 都必须完整的运算出每个元素的返回值并组成一个数组，再做下一个 operator 的运算，我们看下面这段代码</p><pre class="language-javascript"><code><span class="token keyword">var</span> source <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">var</span> example <span class="token operator">=</span> source
              <span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span>x <span class="token operator">=&gt;</span> x <span class="token operator">%</span> <span class="token number">2</span> <span class="token operator">===</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// 这里会运算并返回一个完整的数组</span>
              <span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>x <span class="token operator">=&gt;</span> x <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 这里也会运算并返回一个完整的数组</span>

</code></pre><p>上面这段代码，相信读者们都很熟悉了，大家应该都有注意到 <code>source.filter(...)</code> 就会返回一整个新数组，再接下一个 operator 又会再返回一个新的数组，这一点其实在我们实例 map 跟 filter 时就能观察到</p><pre class="language-javascript"><code>Array<span class="token punctuation">.</span>prototype<span class="token punctuation">.</span><span class="token function-variable function">map</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span>callback<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">var</span> result <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token comment">// 建立新数组</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">forEach</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span>item<span class="token punctuation">,</span> index<span class="token punctuation">,</span> array<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        result<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span><span class="token function">callback</span><span class="token punctuation">(</span>item<span class="token punctuation">,</span> index<span class="token punctuation">,</span> array<span class="token punctuation">)</span><span class="token punctuation">)</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> result<span class="token punctuation">;</span> <span class="token comment">// 返回新数组</span>
<span class="token punctuation">}</span>

</code></pre><p>每一次的 operator 的运算都会建立一个新的数组，并在每个元素都运算完后返回这个新数组，我们可以用下面这张动态图表示运算过程</p><p><img src="http://i.giphy.com/l0HlPZeB9OvFu7QwE.gif" alt></p><p>Observable operator 的运算方式跟数组的是完全的不同，虽然 Observable 的 operator 也都会回传一个新的 observable，但因为元素是渐进式取得的关系，所以每次的运算是一个元素运算到底，而不是运算完全部的元素再返回。</p><pre class="language-javascript"><code><span class="token keyword">var</span> source <span class="token operator">=</span> Rx<span class="token punctuation">.</span>Observable<span class="token punctuation">.</span><span class="token keyword">from</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">var</span> example <span class="token operator">=</span> source
              <span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span>x <span class="token operator">=&gt;</span> x <span class="token operator">%</span> <span class="token number">2</span> <span class="token operator">===</span> <span class="token number">0</span><span class="token punctuation">)</span>
              <span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>x <span class="token operator">=&gt;</span> x <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span>

example<span class="token punctuation">.</span><span class="token function">subscribe</span><span class="token punctuation">(</span>console<span class="token punctuation">.</span>log<span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><p>上面这段代码运行的方式是这样的</p><ol><li>送出 <code>1</code> 到 filter 被过滤掉</li><li>送出 <code>2</code> 到 filter 在被送到 map 转成 <code>3</code>，送到 observer <code>console.log</code> 印出</li><li>送出 <code>3</code> 到 filter 被过滤掉</li></ol><p>每个元素送出后就是运算到底，在这个过程中不会等待其他的元素运算。这就是渐进式取值的特性，不知道读者们还记不记得我们在讲 Iterator 跟 Observer 时，就特别强调这两个 Pattern 的共同特性是渐进式取值，而我们在实例 Iterator 的过程中其实就能看出这个特性的运行方式</p><pre class="language-javascript"><code><span class="token keyword">class</span> <span class="token class-name">IteratorFromArray</span> <span class="token punctuation">{</span>
	<span class="token function">constructor</span><span class="token punctuation">(</span>arr<span class="token punctuation">)</span> <span class="token punctuation">{</span>
		<span class="token keyword">this</span><span class="token punctuation">.</span>_array <span class="token operator">=</span> arr<span class="token punctuation">;</span>
		<span class="token keyword">this</span><span class="token punctuation">.</span>_cursor <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>

	<span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
		<span class="token keyword">return</span> <span class="token keyword">this</span><span class="token punctuation">.</span>_cursor <span class="token operator">&lt;</span> <span class="token keyword">this</span><span class="token punctuation">.</span>_array<span class="token punctuation">.</span>length <span class="token operator">?</span>
		<span class="token punctuation">{</span> value<span class="token punctuation">:</span> <span class="token keyword">this</span><span class="token punctuation">.</span>_array<span class="token punctuation">[</span><span class="token keyword">this</span><span class="token punctuation">.</span>_cursor<span class="token operator">++</span><span class="token punctuation">]</span><span class="token punctuation">,</span> done<span class="token punctuation">:</span> <span class="token boolean">false</span> <span class="token punctuation">}</span> <span class="token punctuation">:</span>
		<span class="token punctuation">{</span> done<span class="token punctuation">:</span> <span class="token boolean">true</span> <span class="token punctuation">}</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>

	<span class="token function">map</span><span class="token punctuation">(</span>callback<span class="token punctuation">)</span> <span class="token punctuation">{</span>
		<span class="token keyword">const</span> iterator <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">IteratorFromArray</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>_array<span class="token punctuation">)</span><span class="token punctuation">;</span>
		<span class="token keyword">return</span> <span class="token punctuation">{</span>
			next<span class="token punctuation">:</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
				<span class="token keyword">const</span> <span class="token punctuation">{</span> done<span class="token punctuation">,</span> value <span class="token punctuation">}</span> <span class="token operator">=</span> iterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
				<span class="token keyword">return</span> <span class="token punctuation">{</span>
					done<span class="token punctuation">:</span> done<span class="token punctuation">,</span>
					value<span class="token punctuation">:</span> done <span class="token operator">?</span> undefined <span class="token punctuation">:</span> <span class="token function">callback</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span>
				<span class="token punctuation">}</span>
			<span class="token punctuation">}</span>
		<span class="token punctuation">}</span>
	<span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token keyword">var</span> myIterator <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">IteratorFromArray</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
myIterator<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span>x <span class="token operator">=&gt;</span> x <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
myIterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// { done: false, value: 2 }</span>

</code></pre><p>虽然上面这段代码是一个非常简单的示范，但可以看得出来每一次 map 虽然都会返回一个新的 oterator，但实际上在做元素运算时，因为渐进式的特性会使一个元素运算到底，Observable 也是相同的概念，我们可以用下面这张动态图表示运算过程</p><p><img src="http://i.giphy.com/3o6ZtqrBfUyHvMDQ2c.gif" alt></p><p>渐进式取值的观念在 Observable 中其实非常的重要，这个特性也使得 Observable 相较于 Array 的 operator 在做运算时来的高效很多，尤其是在处理大量资料的时候会非常明显！</p><h2 id="今日小结"><a href="#今日小结" aria-hidden="true" class="header-anchor">#</a> 今日小结</h2><p>今天我们讲解了 Observable 跟数组各自 operators 运行上的差异，这些细微的差异实际上对程式的运行效率有着很大的影响。</p><p>从我们一开始从数组作为 obsevable 的切入点，中间介绍了各种常用的 operator，到今天我们厘清了数组跟 Observable 运行上的差异，在 Observable 这块我们几乎已经完成了，剩下的是一些衍生出来的东西，像是 multicast, publish... 等，这些我们会在介绍完 Subject 后在做说明！</p></div></div></div><div class="v-card__actions"><div class="flex xs12"><a href="/BLOG/tags/RXJS"><span tabindex="0" class="v-chip capitalize chip-tag v-chip--label v-chip--small"><span class="v-chip__content">RXJS</span></span></a></div></div></article></div><div class="flex text-xs-left xs6"><a href="/BLOG/posts/rxjs22.html" class="post-nav v-btn v-btn--flat v-btn--router"><div class="v-btn__content"><div class="grey--text"><i class="fa mr-1 fa-chevron-left"></i>Prev</div><div class="title mt-1 primary--text hidden-xs-only">22 Subject 基本观念</div></div></a></div><div class="flex text-xs-right xs6"><a href="/BLOG/posts/rxjs20.html" class="post-nav v-btn v-btn--flat v-btn--router"><div class="v-btn__content"><div class="grey--text">Next
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